How to Retire a Chimney
It's no longer in use. Can we just seal it off?
Our old, unlined chimney is no longer safe to use. I would like to seal the top, to keep critters and debris out, and stuff insulation into the lower flue opening. Will that approach work, or would it cause other problems? —Penny Coffey, Batavia, OH
Kevin O’Connor replies: This is a good question for Mark Schaub, a chimney and fireplace expert who has worked on a number of This Old House TV projects. Here’s what he had to say.
“A chimney needs to be able to breathe, even after it ceases to function. It’s okay to seal one end of the flue, either the top or the bottom, but sealing both ends will trap moisture inside and hasten the chimney’s deterioration.
“Your approach will depend on the location of the damper. If it’s at the chimney’s lower end, near the firebox, stuff insulation above the damper area to keep warm air from being lost up the flue. I’d recommend a mineral wool insulation such as Roxul, because it’s hydrophobic and can dry out if it ever gets wet. Also, make sure there’s a chimney cap with mesh screening at the top of the flue to keep out rain, as well as leaves and critters. A stainless-steel cap is the best option—it will last longer and won’t leave rust stains like one made of galvanized steel.
“If the chimney has a damper at its top, close it tightly and leave the rest of the chimney open down to the firebox.
“Now, when you have a woodstove connected to your chimney instead of a fireplace, fill the thimble—the hole where the stove’s flue goes into the wall and connects to the chimney—with mortar and some pieces of brick.
“Follow these steps and the chimney will still be there for the next owners to reline and put to use, if they choose.”
Thanks to Mark Schaub, Chimney Savers, Hillsborough, NJ; 908-359-7798